Hook up ui::Compositor to Display's BeginFrameSource

cc/surfaces/display_client.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CC_SURFACES_DISPLAY_CLIENT_H_
 #define CC_SURFACES_DISPLAY_CLIENT_H_
 
 #include "base/memory/scoped_ptr.h"
 #include "base/time/time.h"
 
 namespace cc {
 
+class BeginFrameSource;
 struct ManagedMemoryPolicy;
 
 class DisplayClient {
  public:
   virtual void CommitVSyncParameters(base::TimeTicks timebase,
                                      base::TimeDelta interval) = 0;
   virtual void OutputSurfaceLost() = 0;
   virtual void SetMemoryPolicy(const ManagedMemoryPolicy& policy) = 0;
+  // This must be non-null and always the same.

[enne (OOO), 2016/03/22 21:05:18]

This has to always the same because the SurfaceDisplayOutputSurface just pulls
this value and has no way of knowing if its changes.

+  virtual BeginFrameSource* BeginFrameSourceForChildren() = 0;
 
  protected:
   virtual ~DisplayClient() {}
 };
 
 }  // namespace cc
 
 #endif  // CC_SURFACES_DISPLAY_CLIENT_H_